The present invention relates to methods and implantable systems to reverse damage to heart muscle following myocardial infarction. Specifically, this involves the repopulation of the damaged myocardium with undifferentiated contractile cells, which may be formed in situ through the use of genetic engineering techniques, and augmentation with electrical stimulation.
Coronary Artery Disease (CAD) affects 1.5 million people in the USA annually. About 10% of these patients die within the first year and about 900,000 suffer from acute myocardial infarction. During CAD, formation of plaques under the endothelial tissue narrows the lumen of the coronary artery and increases its resistance to blood flow, thereby reducing the O2 supply. Injury to the myocardium (i.e., the middle and thickest layer of the heart wall, composed of cardiac muscle) fed by the coronary artery begins to become irreversible within 0.5-1.5 hours and is complete after 6-12 hours, resulting in a condition called acute myocardial infarction (AMI) or simply myocardial infarction (MI).
Myocardial infarction is a condition of irreversible necrosis of heart muscle that results from prolonged ischemia. Damaged regions of the myocardium are infiltrated with noncontracting scavenger cells and ultimately are replaced with scar tissue. This fibrous scar does not significantly contribute to the contraction of the heart and can, in fact, create electrical abnormalities.
Those who survive AMI have an 4-6 times higher risk of developing heart failure. Current and proposed treatments for those who survive AMI focus on pharmacological approaches and surgical intervention. For example, angioplasty, with and without stents, is a well known technique for reducing stenosis. Most treatments are designed to achieve reperfusion and minimize ventricular damage. However, none of the current or proposed therapies address myocardial necrosis (i.e., degradation and death of the cells of the heart muscle). Because cardiac cells do not divide to repopulate the damaged region, this region will fill with connective tissue produced by invading fibroblasts. Fibroblasts produce extracellular matrix components of which collagen is the most abundant. Neither the fibroblasts themselves nor the connective tissue they form are contractile. Thus, molecular and cellular cardiomyoplasty research has evolved to directly address myocardial necrosis.
Cellular cardiomyoplasty involves transplanting cells, rather than organs, into the damaged myocardium with the goal of restoring its contractile function. Research in the area of cellular cardiomyoplasty is reviewed in Cellular Cardiomyoplasty: Myocardial Repair with Cell Implantation, ed. Kao and Chiu, Landes Bioscience (1997), particularly Chapters 5 and 8. For example, Koh et al., J. Clinical Invest., 96, 2034-2042 (1995), grafted cells from AT-1 cardiac tumor cell line to canines, but found uncontrolled growth. Robinson et al., Cell Transplantation, 5, 77-91 (1996),grafted cells from C2C12 skeletal muscle cell line to mouse ventricles. Although these approaches produced intriguing research studies, cells from established cell lines are typically rejected from the human recipient. Li et al., Annals of Thoracic Surgery, 62, 654-661 (1996), delivered fetal cardiomyocytes to adult mouse hearts. They found improved systolic pressures and noticed that the presence of these cells prevented remodeling after the infarction. Although their results showed the efficacy of transplanted cell technology, this approach would not likely be effective in clinical medicine since the syngeneic fetal cardiac tissue will not be available for human patients. Chiu et al., Ann. Thorac. Surg., 60, 12-18 (1995) performed direct injection of cultured skeletal myoblasts to canine ventricles and found that well developed muscle tissue could be seen. This method, however, is highly invasive, which compromises its feasibility on human MI patients.
Molecular cardiomyoplasty has developed because fibroblasts can be genetically manipulated. That is, because fibroblasts, which are not terminally differentiated, arise from the same embryonic cell type as skeletal muscle, their phenotype can be modified, and possibly converted into skeletal muscle satellite cells. This can be done by turning on members of a gene family (myogenic determination genes or xe2x80x9cMDGSxe2x80x9d) specific for skeletal muscle. A genetically engineered adeno-virus carrying the myogenin gene can be delivered to the MI zone by direct injection. The virus penetrates the cell membrane and uses the cell""s own machinery to produce the myogenin protein. Introduction of the myogenin protein into a cell turns on the expression of the myogenin gene, which is a skeletal muscle gene, and which, in turn, switches on the other members of the MDGS and can transform the fibroblast into a skeletal myoblast. To achieve this gene cascade in a fibroblast, replication deficient adenovirus carrying the myogenin gene can be used to deliver the exogenous gene into the host cells. Once the virus infects the fibroblast, the myogenin protein produced from the viral genes turns on the endogenous genes, starting the cascade effect, and converting the fibroblast into a myoblast. Without a nuclear envelope, the virus gets degraded, but the cell""s own genes maintain the cell""s phenotype as a skeletal muscle cell.
This concept has been well-developed in vitro. For example, Tam et al., J. Thoracic and Cardiovascular Surgery, 918-924 (1995), used MyoD expressing retrovirus in vitro for fibroblast to myoblast conversion. However, its viability has not been demonstrated in vivo. For example, Klug et al., J. Amer. Physiol. Society, 1913-1921 (1995), used SV40 in vivo and succeeded in replicating the nucleus and DNA, but not the cardiomyocytes themselves. Also, Leor et al., J. Molecular and Cellular Cardiology, 28, 2057-2067 (1996), reported the in situ generation of new contractile tissue using gene delivery techniques.
Thus, there is a need for an effective system and the method for less invasive delivery of a source of repopulating agents, such as cells or vectors, to the location of the infarct zone of the myocardium.
Many of the following lists of patents and nonpatent documents disclose information related to molecular and cellular cardiomyoplasty techniques. Others are directed to background information on myocardial infarction, for example.
All patent and nonpatent documents listed in Table 1 are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate upon reading the Summary of the Invention, Detailed Description of Preferred Embodiments, and Claims set forth below, many of the systems, devices, and methods disclosed in these documents may be modified advantageously by using the teachings of the present invention.
The present invention provides methods and implantable systems that reverse the damage to necrotic heart muscle following myocardial infarction. Specifically, this involves combining a method of supplying a source of a repopulating agent with a stimulation device. More specifically, this involves the repopulation of the damaged myocardium with undifferentiated contractile cells and augmentation of the newly formed tissue with electrical stimulation to cause the newly formed tissue to contract in synchrony with the heart to improve the cardiac function.
The repopulation of the damaged myocardium with undifferentiated contractile cells can be carried out using a cellular or a molecular approach. Cellular approaches involve the injection, either directly or via coronary infusion, for example, of undifferentiated contractile cells, preferably cultured autologous skeletal cells, into the infarct zone (i.e., the damaged region of the myocardium). Molecular approaches involve the injection, either directly or via coronary infusion, for example, of nucleic acid, whether in the form of naked, plasmid DNA, optionally incorporated into liposomes or other such delivery vehicle, or a genetically engineered vector into the infarct zone to convert fibroblasts, for example, invading the infarct zone into myoblasts. The genetically engineered vector can include a viral expression vector such as a retrovirus, adenovirus, or an adeno-associated viral vector, for example.
Various embodiments of the present invention provide one or more of the following advantages: restoration of elasticity and contractility to the tissue; increased left ventricular function; reduction in the amount of remodeling (i.e., conversion of elastic and contractile tissue to inelastic and noncontractile tissue); and decreased morbidity and mortality.
In one embodiment, the present invention provides an implantable system comprising: a cell repopulation source comprising genetic material, undifferentiated contractile cells, or a combination thereof capable of forming new contractile tissue in and/or near an infarct zone of a patient""s myocardium; and an electrical stimulation device for electrically stimulating the new contractile tissue in and/or near the infarct zone of the patient""s myocardium. An infarct zone of a myocardium can be determined by one of skill in the art. Near the infarct zone means sufficiently close that damage to necrotic heart muscle is realized. Typically, this means within about 1 centimeter (cm) of the edge of the infarct zone.
In another embodiment, the present invention provides an implantable system comprising: a cell repopulation source comprising skeletal muscle satellite cells capable of forming new contractile tissue in and/or near an infarct zone of a patient""s myocardium; and an electrical stimulation device for electrically stimulating the new contractile tissue in and/or near the infarct zone of the patient""s myocardium, wherein the electrical stimulation device provides burst stimulation.
The present invention also provides a method of repairing the myocardium of a patient, the method comprising: providing an implantable system as described above; implanting the cell repopulation source into and/or near the infarct zone of the myocardium of a patient; allowing sufficient time for the new contractile tissue to form from the cell repopulation source; and electrically stimulating the new contractile tissue. Typically, new contractile tissue forms within about 15 days, although electrical stimulation may not be effective for up to about 14 additional days after the contractile tissue forms.